It is possible in the UK, but don't know about other countries.
I currently fly a single-pilot aircraft as an FO, and log my HP time as PICUS with a signature from the Captain in my logbook to verify this.
It is in our Ops Manual that we must fly our missions/aircraft multi-pilot, so the CAA accept the hours can be logged as multi-pilot hours.
I think that it has to say in the Ops Manual that the AC/mission must be flown multi-pilot in order for you to be able to log the hours.
StraightLevel